This invention relates to the printing industry and in particular to digital offset printing and the use of multi-channel imaging units in this field.
Multi-channel scanning systems are used to achieve a higher throughput than single channel systems in many different fields of imaging, including the graphic arts. In many multi-channel scanning systems that use continuous scan motion for both axes, a form of distortion known as an orthogonality error is often generated. Orthogonality errors occur during imaging when the planar axes of the original image are distorted in such a way that the planar axes of the generated image are no longer mutually perpendicular. An example of a situation where this can arise is when a continuously moving imaging head writes a helix instead of parallel circles on a cylinder-based imaging system.
The way in which the problem manifests itself is described schematically in FIG. 1 and FIG. 2A. As may be understood from FIG. 1, an orthogonality error is caused in the image due to the helical scan path followed by multi-channel spatial light modulator (SLM) 4 over printing plate 1 attached to cylinder 2. This results from the continuous relative motion of the cylinder 2, driven by motor 3, and leadscrew 6. FIG. 2A depicts the printing plate 1, now removed from the cylinder 2, and shows the orthogonality error that results from this scanning arrangement. The problem becomes more severe the greater the number of channels in multi-channel SLM 4, because of the increased pitch of the path traversed by the written spiral data tracks 5.
One common solution to this problem is to shift or delay the starting point of each swath 8 scanned on plate 1 in order to keep the image orthogonal. This solution is shown schematically in FIG. 2B. This solution causes the final image 9 to be rotated relative to the plate 1. While this approach is acceptable for a small number of parallel channels, it becomes less acceptable as the width of the swath 8, written by multi-channel SLM 4, becomes wider due to the increase in the number of channels in multi-channel SLM 4.
In the standard approach to printing, printing plates are prepared on a separate plate-setter machine and the plates so obtained are then transferred to the actual printer. It is entirely feasible to implement solutions of the nature of that depicted in FIG. 2B on the plate-setter machine. The final printing plate would then be corrected in this process such that the plate put on the actual printing machine would be orthogonal.
With the advent of Digital-Offset-Press (DOP) technology, it has become technologically feasible to image the plates on the printing machine using multi-channel writing techniques as depicted in FIG. 1. However, DOP technology does not provide a natural opportunity to correct the orthogonality as described above. Since the final writing of the plate occurs on the very press itself, the plate needs to be written to be orthogonal.
The speed and efficiency of on-press imaging systems scale directly with the number of channels employed in the scanning head. As a result there is much pressure to create scanning heads with ever larger channel counts. The company CreoScitex of Burnaby, BC, Canada manufactures and uses spatial light modulators with 240 and more channels and implements them in equipment for the printing industry. With reference to FIG. 1, this trend towards ever-larger multi-channel systems implies that the orthogonality problem becomes severely exacerbated. Simultaneously the problem cannot be addressed by the simple skewing method depicted in FIG. 2.B because the image is written after the plate is mounted.
In the particular case where both the front and the back of a printing medium are printed upon, the orthogonality problem described here becomes acute. In such applications, there is a standard practice to align the lines and columns of print on either side of the medium. So, for example, page numbers on one side of a page will coincide with the page numbers on the back of that page. For obvious geometric reasons the skew on the two sides of the medium will be in opposite directions and the orthogonality error causes a misalignment that is twice as large as that evident from FIG. 2A. The reason for this doubling of the error is the use of the second imaging system which, being on the opposite side of the printed material, causes the error to compound in the opposite direction.
A partial solution could be to develop a xe2x80x9cmirror imagexe2x80x9d imaging system, having the orthogonality error compound in an opposite direction. This solution is, however, not practical as the same printing press will use two different imaging systems and the images, while in register, will not in fact be orthogonal. For reasons of economics and maintenance it is desirable to make the imaging units interchangeable.
While orthogonalization has been treated, for example by Moyroud in U.S. Pat. No. 4,819,018 and Nishikawa in U.S. Pat. No. 4,745,487, the unique image orientation and alignment problems encountered in printing simultaneously on both sides of the medium when the scan process introduces a skewing of the image have not been addressed.
The success of Digital-on-Press technology in the commercial mass printing industry is therefore directly linked to correcting the orthogonality problem that is exacerbated by the ever-larger channel-counts in multi-channel systems that characterize this technology.
The main object of the present invention is to improve registration of images in a double-sided printing arrangement by correcting the orthogonality of images printed on either side of the printed sheets. A second object is to achieve this orthogonality correction using purely electronic means.
The present invention is a method to generate alignable orthogonal images from nonorthogonal scans in a double-sided digital-offset-press arrangement by shifting the image data in the multi-channel modulators in a direction and rate that keeps the images orthogonal. The image data is shifted parallel to the cylinder axes and the amount of shift per cylinder rotation equals the distance the multi-channel modulators move relative to their respective cylinders per rotation of the relevant cylinder. This prevents xe2x80x9cparallelogramxe2x80x9d type distortion in the images and maintains the orthogonality of the original images, thereby allowing the images on the back and front of the printing medium to be aligned. The shift can be done continuously or in discrete steps. The invention is also extendable to two-dimensional modulators such as deformable mirror devices (DMD) and liquid crystal light valves (LCLV).